Crash when logout requested switching to app from background

Steps to repro:

• Start the app in the sim (logged out)
• Log in
• Switch to Settings, Gitter, toggle log out
• Using the app switcher (double tap home, drag up from bottom), switch back to Gitter

Expect:

• User is logged out and presented with login screen

Actual:

• App hits a trap in:

    func deleteAllObjects(_ saveCallback: @escaping SaveCallback) {
        workerManagedObjectContext.perform {
            for entityName in ["Room", "Suggestion"] {
                do {
                    try self.deleteAllObjects(forEntity: entityName, context: self.workerManagedObjectContext)
                } catch {
                    LogError("Failed to delete all objects", error: error)
                }
            }
            self.saveAll(saveCallback)
        }
    }

Related !8 (merged) !6 (merged)